Fix #3908: use the async iterator element type for yield

Yield translation derived its target type only from synchronous enumerable
interfaces, leaving async iterator yields untyped and preserving compiler
boxing casts. Use the element type already recovered by the async decompiler.

Assisted-by: Copilot:gpt-5.6-sol:GitHub Copilot CLI
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 86d2918e-5a24-48b4-9a86-41d331ec3720

ICSharpCode.Decompiler.Tests/TestCases/Pretty/AsyncStreams.cs

@@ -6,6 +6,12 @@ using System.Threading.Tasks;
 
 namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 {
+	public enum AsyncStreamColor
+	{
+		Red,
+		Green
+	}
+
 	public class AsyncStreams
 	{
 		public static async IAsyncEnumerable<int> CountTo(int until)
@@ -85,6 +91,33 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 				await Task.Yield();
 			}
 		}
+
+		public static async IAsyncEnumerable<IAsyncStreamEntry> ConstrainedGenericYield<T>(IAsyncEnumerable<T> items) where T : IAsyncStreamEntry
+		{
+			await foreach (T item in items)
+			{
+				yield return item;
+			}
+		}
+
+		public static IEnumerable<object> BoxedValues()
+		{
+			yield return true;
+			yield return 'a';
+			yield return AsyncStreamColor.Green;
+		}
+
+		public static async IAsyncEnumerable<object> BoxedValuesAsync()
+		{
+			yield return true;
+			await Task.Yield();
+			yield return 'a';
+			yield return AsyncStreamColor.Green;
+		}
+	}
+
+	public interface IAsyncStreamEntry
+	{
 	}
 
 	public struct TestStruct

ICSharpCode.Decompiler/CSharp/StatementBuilder.cs

@@ -433,7 +433,8 @@ namespace ICSharpCode.Decompiler.CSharp
 
 		protected internal override TranslatedStatement VisitYieldReturn(YieldReturn inst)
 		{
-			var elementType = currentFunction.ReturnType.GetElementTypeFromIEnumerable(typeSystem, true, out var isGeneric);
+			var elementType = currentFunction.AsyncReturnType
+				?? currentFunction.ReturnType.GetElementTypeFromIEnumerable(typeSystem, true, out _);
 			var expr = exprBuilder.Translate(inst.Value, typeHint: elementType)
 				.ConvertTo(elementType, exprBuilder, allowImplicitConversion: true);
 			return new YieldReturnStatement {